1. Field of Invention
This invention relates generally to electronic assemblies and more specifically to thermal management in an electronic assembly.
2. Discussion of Related Art
Many modern electronic systems are constructed by attaching electronic components to printed circuit boards. The printed circuit boards are then installed into a mechanical chassis (i.e. subassembly), which allows them to be installed into a larger frame. In some cases, multiple boards are installed and electrically interconnected within a chassis. The frame, also called the “rack”, provides the mechanical support for each chassis.
The frame also allows each chassis to be electrically interconnected. The boards in one chassis can be electrically interconnected to the boards within other chassis through the same frame. In the most common frame configurations, the printed circuit boards are installed parallel to the ground. The common alternative is to have them installed vertically.
Many electronic manufacturers have agreed on standards for the dimensions of the frames, and also the chassis. In this way, the printed circuit boards installed in the frame can be made by many different companies, but can all easily be included in an assembly. Standardization allows an original equipment manufacturer making an electronic system to buy printed circuit boards from multiple sources to either provide special functionality or to take advantage of economies of scale that result when a supplier sells standard sized components to many manufacturers.
As an example, a rack might be 19 inches (46.55 cm) or 23 inches (56.4 cm) wide. Also, the rack might be defined with units of height, such as 1.75 inches (4.3 cm). Preferably, the opening in the rack is between 18 and 24 inches wide and less than 9 inches in height. These units are sometimes called “rack units.” As used herein, a “rack unit” defines the smallest distance in a particular dimension that can be allotted to any subassembly or chassis. Subassemblies may be taller than one rack unit, but when installed in a frame, space for the subassembly is allocated in integer multiples of rack units. The Enterprise Applications Integration (EAI) organization sets many of these definitions and further examples can be found on their web page at www.eaiindustry.org.
Cooling challenges can arise with rack-mount multi-board chassis or sub-assemblies, because of their limited cooling capacity. For example, electronic systems generally become more powerful (e.g. faster) as each new product is created. More powerful electronics generally require more electrical power and consequently generate more heat. This trend of increasing power is most commonly observed with computer central processors.
However, end users or customers expect that old electronics can be swapped for new ones without requiring any impact on the electrical, mechanical or thermal subsystems. For example, in the case of multi-board chassis or subassemblies, customers expect that new boards fit into the same slot as the predecessor board. In single board electrical systems, customers expect that new computer processors, which are typically the highest power and power density electronic component, will be a direct replacement for an existing processor. As a result, in both cases, the new electronics must be packaged in the same volume as the ones they replace. Miniaturization of electronic circuitry often makes it possible to build new and more powerful electronics in the same volume as was occupied by an earlier less powerful system or component.
However, the amount of power generated by powerful electronic circuits in constrained volumes can be a problem. As more heat is generated in the same volume, the heat density increases. As the heat density increases, the temperature of the circuitry is likely to rise. As the temperature rises, the electronic components might malfunction with greater frequency, prematurely fail, or fail to operate with their intended performance.
It is likely that over the next few years, the amount of power generated by a state of the art computer board will more than double. We have recognized that for sophisticated electronic circuitry that already generates large amounts of power, it is likely that traditional direct air-cooling techniques, using for instance cooling fans, will not adequately cool the subassemblies mounted in racks. We have recognized the benefits of providing a means to transport a portion or all of the heat to another physical location where it is easier to dissipate the heat to air, because of the greater amount of available space.
Methods for transporting liquid significant distances and then dissipating the heat are known. For example, a common one being exploited in electronics cooling is single-phase liquid cooling where a device called a “chiller” is mounted in the rack. The chiller circulates fluid through hoses to a “cold plate” that rests over the electronic circuitry in a subassembly with a high power density. As the electronic circuitry generates heat, the cold plate absorbs the heat. The heat is then absorbed by the fluid circulating inside the cold plate.
The circulating fluid is returned to the chiller where it is pumped through a radiator. Fans induce airflow through the radiator, which dissipates the heat. In this way, the heat is dissipated without an undesirable increase in the temperature of the high power circuit.
It would be desirable if an improved chiller were available that is suitable for use in electronic assemblies.